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DO winglets have effect on Vref when landing?

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DO winglets have effect on Vref when landing?

Old 13th Apr 2014, 08:17
  #61 (permalink)  
 
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Regarding VMCL: (which is really just another form of VMCA but in a differrent phase of flight)

JAR/FAR 25.149 Minimum control speed
(f) VMCL, the minimum control speed during approach and landing with all engines operating, is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5o. VMCL must be established with:
• The aeroplane in the most critical configuration (or, at the option of the applicant, each configuration) for approach and landing with all engines operating;
• The most unfavourable centre of gravity;
• The aeroplane trimmed for approach with all engines operating;
• The most unfavourable weight, or, at the option of the applicant, as a
function of weight.
• Go-around thrust setting on the operating engines

(g) For aeroplanes with three or more engines, VMCL-2, the minimum control speed during approach and landing with one critical engine inoperative, is the calibrated airspeed at which, when a second critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with both engines still inoperative, and maintain straight flight with an angle of bank of not more than 5 degrees. VMCL-2 must be established with [the same conditions as VMCL, except that]:
• The aeroplane trimmed for approach with one critical engine inoperative
• The thrust on the operating engine(s) necessary to maintain an approach path angle of 3 degrees when one critical engine is inoperative
The thrust on the operating engine(s) rapidly changed, immediately after the second critical engine is made inoperative, from the [previous] thrust to:
- the minimum thrust [and then to] - the go-around thrust setting

h) In demonstrations of VMCL and VMCL-2, ... lateral control must be sufficient to roll the aeroplane from an initial condition of steady straight flight, through an angle of 20 degrees in the direction necessary to initiate a turn away from the inoperative engine(s) in not more than 5 seconds.”
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Old 13th Apr 2014, 14:09
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Minimum control speed and stall speed

Whereas the stall speed varies approximately as the square root of weight, the change of minimum control speed with weight is very small. The reason that it changes at all is the bank angle of 5 degrees allowed in the determination. So if someone says that at a particular weight the minimum control speed is less than the stall speed, he is comparing the stall speed at that weight to the minimum control speed established at a lesser weight and extrapolated to the actual weight.

Another aspect to consider is that the stall speed is determined with power off, whereas Vmc is determined with TOGA thrust on the operating engine(s). Since a component of the thrust contributes to the lift, it is in fact possible to demonstrate Vmc below the power-off stall speed.

Last edited by HazelNuts39; 13th Apr 2014 at 14:33.
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Old 13th Apr 2014, 15:34
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As you see from the requirements vmca is determined at the maximum sea-level take-off weight.

What do you mean with: he is comparing the stallspeed at that weight to the minimum control speed established at a lesser weight extrapolated to the actual weight?

An aircraft will also stall at full-power, so why do you want to compare vmca at full power with a stallspeed without power?

The entire point of VMCA is to demonstrate the ability to maintain direction (even though you heading can still change up to 20 degrees) after becoming airborne with the most critical engine failed, the other engine(s) at max to power and a bank maximum bank of 5 degrees.
And thats about as far as VMCA goes. Sure you can demonstrate that in the clean configuration at FL200 you can put TOGA on one engine and then stall it, but thats not the definition of VMCA as required by FAR/JAR. It would be another variant of VMC, like for example VMCL.

just my 2 cents

I realise how you can theoretically, and even practically if it wasn't for the ground being so damn close, get a lower VMC then stallspeed. For me it just has very little to do with the VMCA requirements.

Last edited by 737Jock; 13th Apr 2014 at 15:45.
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Old 13th Apr 2014, 16:08
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Just been looking through minibus fcom.

Stallspeed vs1g for A319 at 60T 1+F is about 112kts at 0 pressure altitude
The minimum V2 speed for take-off limited by VMU/VMCA is 128kts.
Minimum VR is a conservative 113kts.

Stallspeed vs1g for A319 at 60T conf 3 is about 106kts at 0 pressure altitude
The minimum V2 speed for take-off limited by VMU/VMCA is 119kts.
Minimum VR is a conservative 113kts

Can't find any exact VMU values unfortunately. But obviously VR is limited by VMU, so I guess the higher V2 limit must come from VMCA.

Anyway I have the idea these values are all not that far apart. Certainly not as comfortable as mustafagander claims:

For a normal conf 1+f takeoff, a 112kts Vs1g would dictate a V2 of at least 112 x 1.13= 126kts
Yet the minimum V2 for VMU/VMCA is 128kts. (but VR is ok at 113kts)

For conf 3 both are 119kts

Last edited by 737Jock; 13th Apr 2014 at 16:44.
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Old 13th Apr 2014, 18:44
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Originally Posted by 737jock
As you see from the requirements vmca is determined at the maximum sea-level take-off weight.
You are misreading the requirements. Even though Vmca varies little with weight, it is usually determined at all weights where it may be limiting V2, which is usually at the low end of the take-off weight range. It may optionally be determined at the minimum take-of weight because Vmca decreases with increasing weight if a 5 degree bank is used.

JAR/FAR 25.149(c)(4) states an additional requirement, i.e. that Vmca may not exceed 1.13 Vsr at the Vsr for maximum sea-level take-off weight (or any lesser weight necessary to show Vmc).

Last edited by HazelNuts39; 13th Apr 2014 at 18:55.
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Old 13th Apr 2014, 22:00
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The entire point of VMCA is to demonstrate the ability to maintain direction

If I may disagree .. maintenance of controlled flight and heading is fine .. but the underlying reason for establishing Vmc is to draw in another boundary for V2.

For routine operations, we have no business playing with Vmc ... one should leave it to the TPs to frighten themselves from time to time ..
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Old 14th Apr 2014, 06:46
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If I may disagree, first and foremost it's good to know at what airspeed the aircraft can safely get airborne and control it's direction. If you can't do that then there is not much point in having a V2 speed.
If we then use this information together with other data to be able to determine a safe V2 speed that provides a safety margin on various elements while providing us with a positive climb gradient.

Anyway it's a bit of a moot point, chicken or egg discussion.
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Old 14th Apr 2014, 07:18
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The airplane doesn't know what the wind is...
A crusty old captain once asked me why on an out-and-back two leg flight with strong winds, it takes more total time than the same flight in calm winds.

I gave him the right answer but he corrected me saying that when flying into the wind, the wind blows against the blunt leading edge of the wing having a significant effect and when flying in a tail wind the wind blows on the sharp trailing edge and therefore has a lesser effect. So you don't gain back what you lost in the headwind.

He also explained why one leg of a V formation of migrating geese is invariably longer than the other.
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Old 14th Apr 2014, 07:30
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He also explained why one leg of a V formation of migrating geese is invariably longer than the other.
Is it because geese, like pilots, like to keep traffic on their left? I wonder if the ponds they land in are more likely to be left-hand circuits and if they have appropriate noise abatement & "dump" procedures. (I'm actually curious about his answer to the V formation leg thing.)

Maybe he was just being silly to see if you'd buy it, like misd-agin was about the cross-wind LNAV. (I admit, I thought he was serious at first...)
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Old 14th Apr 2014, 07:41
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It's because like cyclists they change the lead. But instead of dropping back straight to the back, they dropout to the side (creating the v-formation) and then crossover to the other side. Which means that one of the legs will be longer.
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Old 14th Apr 2014, 07:56
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A crusty old captain once asked me why on an out-and-back two leg flight with strong winds, it takes more total time than the same flight in calm winds.
As an aside, I was playing with Foreflight and I was looking at flying from Toronto to Buffalo following the lake shore (basically a C shape with 1 leg longer than the other). Outbound wind was calculated to average 12 knots headwind. Return was average 2 knots tail wind. Same altitude, same route.

As an experiment, I plotted direct from airport to airport, and I got 16 knots headwind on the way out, 10 knots tail wind on the way back.

I'm so confused.
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Old 14th Apr 2014, 08:38
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whats foreflight?
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Old 14th Apr 2014, 14:41
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whats foreflight?
Foreflight is an iPad/iiPhone flight planning (and enroute) software. It's really popular in North America, and especially in Canada as it's the only one that has charts for Canada. For us Canadians, it's paper or Foreflight, no other choices.
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Old 14th Apr 2014, 14:48
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He also explained why one leg of a V formation of migrating geese is invariably longer than the other.
It is simply because geese are in pairs, so with a leader, one leg will invariably be longer....
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Old 15th Apr 2014, 10:20
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mustafagander
I have produced below an extract from Airbus magazine on VMC tests conducted for the A380. It does not give the impression that VMCA is below Vs. I am not saying the two have direct connection but you cannot fly below Vs.
VMCA, VMCL,
When engines and systems are configured, we start about 20 kt above the predicted value, then, we decelerate slowly keeping heading constant. Necessary rudder increases as the speed decreases, eventually up to the stop. Further deceleration will need some bank to still keep the heading constant.
The “true” VMCA is obtained when the bank angle reaches 5° in the opposite sense to the “failed” engine. This bank angle is very important as it allows a further speed reduction of about 5 to 10 kt, compared to the same test performed with wings levelled. Where is this strange rule coming from? It is a mystery

The tests to obtain VMCL and VMCL-2 are similar. But there is more to do. A demonstration that the roll manoeuvrability at VMC is sufficient must be performed.
How do you perform this demonstration when below stall speed?






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Old 15th Apr 2014, 10:30
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vilas,

Were your attachment to become visible to me, I would not be too interested in what it shows. I have been on about BOEINGs, in particular B744. I have quoted from their certified Performance Limitations Manual figures extracted from the graphs therein.

Airbus may have an entirely different certification regime, I know not.

As I have said, I wonder how such speeds below Vs1g can ever be demonstrated, but there we have it - part of the certified data package. I also mentioned the PA44 which has Vmca below Vs1g under certain conditions and a prohibition on intentionally operating the aircraft below the "intentional single engine speed" as extracted from the graphs in the POH.

The bottom line for B744 is that Vmca is below Vs1g for all usable T/O weights. Vmca is also below Vmcg which can be a comfort with a V1 cut.
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Old 15th Apr 2014, 11:03
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what you quoted are irrelevant numbers that do not correlate in aircraft configuration, the only correlation is that they come from the same manual. But the qouted stallspeed is in clean configuration, not the required take-off configuration.

Please provide some correlating numbers. Instead talking about how they are too complicated.

I showed numbers from the performance section of the minibus. Although they are certainly not exact, it does not show the massively comfortable margins you claim.
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Old 15th Apr 2014, 11:16
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http://www.avioconsult.com/downloads...%20failure.pdf

When banking, a component of the weight, leads to a side force due to bank angle (Wsin φ in Figure 2), that can re- place the side force Yβ due to sideslip that was required for balance with the wings level. The small bank angle decreases the sideslip angle to a minimum, decreasing the total drag and, hence, increasing climb performance. Side force Wsin φ acts in the centre of gravity and therefore does not cause any ad- verse yawing moments.
Because the side force Yδr, generated by the vertical tail with rudder, no longer has to act against the side force due to side- slip Yβ, but only against the thrust yawing moment NT, the rudder deflection need not be maximal, the airspeed can be reduced until the deflection is again maximal, or the vertical tail can be dimensioned smaller to save manufacturing cost and weight
and weight.
FAR/CS 23.149 and 25.149 allow the engineer designing the vertical tail to use a bank angle of maximum 5 degrees. Reducing the size of the verti- cal tail increases VMCA (for a high enough side force Yδr). FAR/CS 23.149, however, does not allow the vertical tail to be made so small that VMCA exceeds 1.2 times the stall speed (VS). Hence, the verti- cal tail is made just big enough to maintain straight flight while the thrust of the opposite engine is at the maximum takeoff setting, the rudder is maximal deflected and while maintaining a small bank angle as opted by the designer of the vertical tail, usually between 3 and 5 de-
Sorry but I cannot find any evidence at all that VMCA would massively be below Vs in the configuration its actually tested in. And the only reason for that would be the lack of pilots banking into the live engine. Its a crazy notion that basicly would mean the tailfin is way too big, thus weight materials can be saved by making it smaller. While also reducing fuel burn.

Now since they build entire airplanes made from carbon, to save a bit of fuel, do you seriously consider that they would stick on a tailfin that is too big?

I don't really care if its theoretically possible. It would not be economical and adjusted as soon as they notice it.

Sure if you design a longer aircraft based on a basemodel the tailfin could be made smaller (usually they won't), as the arm becomes longer. But the entire idea of super big margins is just ludicrous.

Last edited by 737Jock; 15th Apr 2014 at 11:38.
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Old 15th Apr 2014, 11:55
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737jock, if I understand your post, I think you haven't considered all the variables of configuration. In particular, flap changes Vs a lot but doesn't change Vmc much.

The tailfin may be perfectly sized for controlling the aircraft for an asymmetric go around with landing flap at minimum weight - that is, in a combination of circumstances giving a very low stall speed with a moderate to high Vmc.

But in that case, Vmc will be very much lower than required (for certification) in clean or take-off configurations. It could well be below Vs.

If I've misunderstood your post, my apologies!
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Old 15th Apr 2014, 11:58
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maybe if you consider that vmcA is only valid in the take-off configuration you would understand my post.

vmc is a term that is way too generic, every phase and configuration has its own vmc. For takeoff its called vmcA, for approach/landing/go-around its vmcL, on the ground vmcG.
And you could make up a whole bunch more for other regimes.

You have to compare the applicable vmc, to the stallspeed that applies to the configuration and flightphase the aircraft is in.

The most likely explanation for a VMCA below vs is that the aircraft has been lengthened and the tailfin kept the same size for commonality. For example the a318 has a latger tailfin then the other minibuses to compensate for the shorter fuselage (arm). But still it won't be more then a few knots.

In any case I doubt it because nowhere in flighttesting do I see statements where testpilots put the aircraft into a stall in order to determine VMCA.

But as I have said theoratically you could have a vmca of almost zero if your tailfin and rudder is infinitely big or the tail infinitely long.

BTW stallspeed doesn't change that much between configurations, from clean to the first configuration the change is massive, thereafter its effect is much smaller.

Last edited by 737Jock; 15th Apr 2014 at 12:14.
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